ESR1 and breast cancer: Based on these markers, breast cancer molecular subtypes are classified as follows: luminal A (ER-positive and/or PR-positive, HER2-negative and Ki67 low), luminal B (ER-positive and/or PR-positive, HER2-negative and Ki67 high), luminal-HER2 (ER-positive and/or PR-positive and HER2-positive), HER2-enriched (ER-negative, PR-negative, HER2-positive), basal-like (ER-negative, PR-negative, HER2-negative, and EFGR-positive or CK5/6-positive), and triple-negative phenotype (TN) (ER-negative, PR-negative, HER2-negative).